1. Field of the Invention
This invention relates to an air diffuser, and more particularly to an improved air diffuser which is suitable for use in an aeration tank for waste water treatment.
2. Description of the Prior Art
A typical air diffuser which is used in an aeration tank for waste water treatment or the like comprises an air chamber having an orifice and an air diffuser plate disposed at the top portion thereof. The air diffuser plate of this type has been formed by bonding such minute particles as silica sand or finely pulverized particles of ceramic materials or synthesized resin fine particles, with any of adhesives suitable for producing the porous structure. Accordingly, in the air diffuser plate of the prior art, the shape of the pores for air diffusion is not uniform and the size of the pores varies depending on the distribution of minute particles and the degree of densifying of the minute particles, so that the diameters of the pores vary considerably and the air-pressure loss at the diffusing pores is not uniform. Besides, the average diameter of the air diffusing pores is fairly large, and the air diffusing pores with a large diameter and a small air-pressure loss will allow passage of a large amount of air and generate a large quantity of coarse air bubbles. On the other hand, the air diffusing pores with a small diameter and a large air-pressure loss allow passage of only a small amount of air or hardly allow passage of air, so that such air diffusing pores produce only a comparatively small quantity of fine air bubbles. Consequently, the air diffuser plate of the prior art has a shortcoming in that its efficiency is very low. Even when fine bubbles are produced from a diffusing pore of which the spacings between adjacent ones are not uniformly located, the fine air bubbles tend to combine with other fine air bubbles from closely adjacent diffusing pores, so as to turn the fine air bubbles into coarse air bubbles.
In recent years, attention has been called to air diffusers having small pores with uniformly fine particles of ceramic material from the standpoint of improving the air diffusing efficiency, and an example of such air diffuser has been disclosed by D. H. Houck and A. G. Boon in the "Survey and Evaluation of Fine Bubbles Dome Diffuser Aeration Equipment", (EPA-600/S2-81-222, October, 1981). The dome diffuser of Houck and Boon uses a maximum air diffusing pore diameter of 150 .mu.m so as to achieve a high air diffusing efficiency. However, the dome diffuser has a shortcoming in that slime is apt to deposit on the surface thereof. More particularly, there exists a distribution of the air-pressure loss in the dome diffuser of the above-referred type, so the activated sludge in the tank deposits on the surface of the dome diffuser and also slime piled up by biological breeding occurs which accelerates the clogging of the diffusing pores with a low level of air diffusion therethrough. Because, the dome diffuser has an irregular distribution of the air-pressure loss due to the pore size distribution the finer pores, which have a larger air-pressure loss, will be hampered or blocked by activated sludge in the treated waste water. Furthermore, when the compressed-air is not transmitted into the dome diffuser, the activated slime deposits and piles at the surface to cause the holes to be plugged.
It is also known to mount a plurality of such porous air-diffuser plates on the top of a shallow box or to dispose a plurality of the porous of the porous plates on a box-type holder acting as an air supply duct as in the case of a planar air diffuser, so as to form an air diffuser for diffusing air through the thus disposed porous plates. In this case, non-uniform pressure loss in the porous plates is inevitable, so that the overall bubbling or foaming of the air diffuser becomes uneven. Even if the pressure loss of the individual porous plates are adjusted by orifices or the like, the overall pressure loss of the air diffuser fluctuates and uniform foaming cannot be expected in the air diffuser, so that the air diffusing efficiency is low and the slime is apt to deposit on the surface of the porous plates of this box-type air diffuser.